Andrew stopped Gary King from drinking the pint because he knew that once the pub crawl was campleted, Gary wouldn’t have anything left to live for. Literally, it would be his own personal World’s End. This ties into the films last scene where Gary King is seen, once again, recreating the pub crawl.

I don’t think Gary turned evil at the end. I think at the end Simon Pegg’s character decided to head into the lawless barbaric north (you know, Scotland) to conquer it. I think Scotland had become like New York from the Plisken series (and Doomsday) where everyone exiles the outlaws and criminals.

Either that or he and his robot buddies became a combination of roving minstrels and Judge Dredd-esque lawgivers. I think the writers were trying to foreshadow it with his ramble about King Arthur, the reminiscing about their band, and him wanting a purpose in life, and of course his name. And I think he started shaving because he sees it as a challenge — and symbolically as a sign that he’s cleaned up his act.

The alien was a bit of jerk, frying out all of technology as he leaves. It’s like flipping over the game-board.